On Tue, 25 Nov 2003, r norman wrote:
> On 24 Nov 2003 22:06:58 -0800, rgregoryclark at yahoo.com (Robert Clark)
> wrote:
>> >Has there been any investigation of the possibility that the origin of
> >the mitotic spindle really is due to electromagnetism?
Not as far as i know.
> There is no reason to believe magnetism is a mechanism just because
> the pictures look similar. Look at the electric field of a dipole
>http://hyperphysics.phy-astr.gsu.edu/hbase/electric/dipole.html to see
> the same thing.
That's right, and there are probably other examples.
> The similarty in shape is caused by processes which share some
> superficial similarities but major differences
Ah, now i'd say that here we have processes which share some major
similarities but have superficial differences :). The two systems are
entirely different at the level of specifics - proteins in a cell versus,
er, vibrations in the ether or whatever it is now. However, i have to
wonder, in an ignorant, D'Arcy-Thomsonesque way (if only!), if the two are
actually governed by very similar general organising principles, whose
similarity leads to the similarity of the forms.
The magnetic thing is about small elements (the iron filings) polarising
and linking up to form a polar filament under the influence of an
organising field, which is established as a tension between two poles, and
orienting themselves at every point so that they lie in the direction of
the greatest gradient. The cellular example is, to me, tantalisingly
similar - obviously, the spindle is homopolar (the two poles are of the
same type, rather than north-south opposites), but the microtubules are
indeed polar chains of polar elements.
Now, i'm not saying that tubulin monomers behave anything like iron
filings - they grow from a point, rather than spontaneously aggregating -
but the idea of a field of some quantity (eg activity of a kinase or
G-protein), defined by the interaction of the centromeres and governing
the form of the spindle, is an interesting one. Since the spindle is
homopolar, the true poles of the field would probably have to be, on the
one hand, the centromeres, and, on the other, the metaphase plate.
Although that just begs the question of how the position of the metaphase
plate is defined!
Anyway, it's all a very interesting, and poorly understood, complex of
problems - just the sort of thing we like!
> In the spindle, the process is very different. The spindle fibers
> radiate outward from both poles. Some of these, the astral fibers,
> always remain that way. These do not look at all like the magnetic or
> electric dipole lines but look more like a magnetic or electric
> monopole. Others meet (either by direct contact as in the polar
> fibers or by connecting to the same chromosome in the kinetochore
> fibers). These fibers tend to spread out from one pole and then
> rejoin at the other, and so sort of look like the field lines of the
> magnetic or electric dipole. Since the fibers that attach to the
> chromosomes are the "important" ones, they are the ones shown in all
> the diagrams.
Ah, but if that was all it was, the spindle fibres would be straight,
wouldn't they? They aren't, they definitely curved.
> So in this case, the similarity is really superficial. However, the
> cause of science is always furthered by trying to find relationships
> between seemingly very different things -- so keep looking and keep
> asking!
I agree wholeheartedly! And apologise for my slightly barming rambling.
tom
--
Tom Anderson, MRC Laboratory for Molecular Cell Biology, UCL